1. Field of the Invention
The present invention relates to hybrid drive systems in general and, more particularly, to a hybrid drive system of a motor vehicle including a combination of a hydraulic transmission and an electric drive.
2. Description of the Prior Art
Hybrid hydraulic (or regenerative) drive systems are known and have been applied to motor vehicles, especially trucks and buses. Such systems harness energy by retarding the vehicle under braking conditions and accumulating that energy for later use to propel the vehicle. More specifically, typical hybrid hydraulic drive systems utilize a hydraulic machine in the form of an integrated hydraulic pump/motor unit to absorb and add power to a conventional vehicle drive system. While the system is absorbing power the hydraulic pump/motor unit is used to pump hydraulic fluid from a low-pressure vessel or reservoir into a hydraulic energy storage system. This energy storage system typically consists of one or more nitrogen charged hydraulic accumulators. In many cases the functionality also exists to absorb power and reject it into the atmosphere as waste heat energy rather than capturing it. When the hybrid hydraulic system is adding power to a driveline of the vehicle drive system, the hydraulic pump/motor unit is acting as a motor, utilizing the hydraulic energy stored in the accumulator(s).
The current systems have however lacked flexibility in their operation, as they principally have been arranged to dump accumulated energy all at once, for example when a vehicle is accelerating from a standing start, while those systems that have allowed for more controlled release of stored energy, have not done so to optimum efficiency. The use of the energy in the known systems is therefore somewhat inefficient and the known systems therefore have not met with widespread use. Additionally, known systems are time consuming and labor intensive to install.
The motor/pump units in the current hybrid hydraulic drive systems are coupled to the driveline of the vehicle drive system downstream of a vehicular transmission. In other words, the driveline integrated pump/motor absorbs and adds hydraulic power after the vehicular transmission. Due to the reduction gear ratio of the vehicular transmission and the inherent frictional and other power losses in the transmission, the motor/pump unit of the current hybrid hydraulic drive systems should generate substantial amount of the pressurized hydraulic fluid. Single speed driveline integrated systems are limited in their maximum power contribution by the speed of the driveline. Typically when the vehicle speed is low, the driveline speeds are low, this results in low power availability to the driveline. The only way to compensate for the lower driveline speeds is to use a larger displacement pump/motor unit. The larger pump/motor unit allows for a higher torque to be applied to the driveline. For a single speed driveline integrated design, the power availability differential can only be partially mitigated through the use of a larger pump/motor. At some point, depending on the particular application, it becomes unreasonable to increase the size of the pump/motor. Another important factor is that in most cases the higher the pump/motor displacement, the lower the maximum permissible operating speed. This lower maximum operating speed often reduces the operation range of the entire hybrid hydraulic system and thus reduces system effectiveness.
In most cases, multi-speed driveline integrated designs help to reduce the limitations indicated above. However, the additional cost, complexity and weight of such multi-speed designs detract from the feasibility of multi-speed designs. Ultimately the functionality of such multi-speed interface systems duplicates the functionality of the vehicle transmission and is therefore redundant.
Therefore, the motor/pump unit of the current driveline integrated hybrid hydraulic drive systems is relatively large in size, heavy and bulky.
Accordingly, it is the intent of this invention to overcome these shortcomings of the prior art, and more specifically, to overcome specific packaging and performance limitations of a driveline mounted regenerative drive system.